The present invention is directed to a rapidly modulatable laser diode having a flat frequency curve at the modulation amplitude.
The direct frequency modulation of semiconductor lasers is a promising method for message transmission in optical communication networks that is particularly useful for coherent transmission. In its simplest form, the frequency modulation is produced by varying the laser current. This effects a variation of the charge carrier density in the active zone of the laser diode and, thus, effects a variation of the emission frequency via a change in the optical refractive index in the laser resonator. However, due to the dissipated heat involved therewith a change in the current simultaneously results in a temperature change which also influences the emission frequency. This influence is greater than that of the charge carrier and is oppositely directed. Since the temperature change is limited to frequencies below 10 MHz because of the relatively large thermal time constant, there results a dependency of the modulation amplitude on the modulation frequency whereby a minimum of the amplitude as well as a phase reversal occur at approximately 1 through 10 MHz.
In prior art modulatable laser diodes for high frequencies the refractive index is not varied by the laser current but by one or more additional currents or by utilizing the field effect instead of the injection of charge carriers. Laser modulator structures that utilize the effect of charge carrier injection are, first, the DFB laser (distributed feedback) that is multiply divided in length and, second, the TTG-DFB laser diode (tunable twin guide).
Given three-section DFB lasers, the intensities of the currents are combined in the different sections such that the disadvantageous effect of the dissipated heat is compensated. Since the charge carrier density in all three sections is varied within the laser-active layer in which the life expectancy of the charge carriers is extremely short due to the induced recombination, these three-section DFB lasers can be modulated up to frequencies far above 2.5 GHz. The considerable measurement and control outlay that is required for identifying and observing the suitable operating points of these lasers is nonetheless disadvantageous.
In the TTG-DFB laser, the frequency is modulated by charge-carrier injection into a separate tuning layer that extends over the entire laser length. Since only one current has to be varied and the laser current is fundamentally independent of the modulation, the modulation is thereby substantially simpler in technological terms than for three-section DFB lasers. The disadvantage of charge carrier injection into a separate layer is the inherent limitation of the modulation frequency to values below 500 MHz. This is caused by the life expectancy of the charge carriers that is substantially longer in the separate layer than in the active layer.
Modulation of the light due to the field effect in potential well structures (quantum confined Stark effect) is more beneficial than modulation on the basis of charge carrier injection both in view of the maximum modulation frequency as well as in view of the frequency-independent curve of the modulation amplitude. In prior art designs, the laser and the modulator layers are integrated longitudinally relative to one another. This is complicated in terms of manufacturing technology since the required potential well structures must selectively grow epitaxially outside the actual laser region. Disturbing variations in the layer thickness due to existing surface structures are thereby difficult to avoid.
Given the laser structure disclosed in the publication by T. Wolf et al., "Tunable Twin-Guide Lasers with Flat Frequency Modulation Response by Quantum Confined Stark Effect" in Appl. Phys. Lett. 60, 2472-2474 (1992), the field effect in the tuning layer of a TTG-DFB laser is utilized for frequency modulation. The tuning layer is thereby composed of a sequence of potential well layers (quantum well). Differing from conventional TTG-DFB lasers, this laser is also designed for minimum parasitic capacitances and inductances. As a result thereof, the modulation amplitude becomes frequency-independent and the maximum modulation frequency lies far above 2.5 GHz. The potential well structures are simpler to produce given transversal integration in the TTG-DFB laser than given a longitudinal integration.